Radio frequency (RF) equipment uses a variety of approaches and structures for receiving and transmitting radio waves in selected frequency bands. Typically, filtering structures are used to maintain proper communication in frequency bands assigned to a particular band. The type of filtering structure used often depends upon the intended use and the specifications for the radio equipment. For example, dielectric and coaxial cavity resonator filters are often used for filtering electromagnetic energy in certain frequency bands, such as those used for cellular and PCS communications.
With today's radio signal systems, intermodulation performance is becoming increasingly significant. The air is filled with radio waves from a myriad of sources, including cellular telephones, radio stations, radar systems, and satellites. Viewed simply, intermodulation results in undesired radio signals that interfere with desired radio signals. Sometimes desired radio signals combine to create intermodulation. An example serves to illustrate the problem created by intermodulation.
An example radio receiver includes a bandpass filter and a low noise amplifier to provide as output a desired frequency range of radio signals, such as, 898.2 MHz-900.8 MHz. Thus, radio signals outside the range, or "passband," are filtered out of the radio signal. Often, however, two radio signals whose frequencies are close to the passband are captured by the receiver. Two example signals have frequencies of 896 MHz and 897 MHz. It will be appreciated that the signals having frequencies of 896 MHz and 897 MHz are outside the passband. However, the two signals combine to produce third order intermodulation signals having frequencies of 895 MHz and 898 MHz. The 895 MHz signal falls outside the passband, and the 898 MHz signal falls at the edge of the passband. It will be appreciated that other pairs of signals may produce third order intermodulation signals that are within the passband.
At low power levels, intermodulation may not present an interference problem. However, when power is increased, the level of the third order intermodulation signals increases at a rate three-times that of the first order signals. Thus, if increased power results in the 896 MHz and 897 MHz first order signals being increased by 1 dB, the 898 MHz third order signal increases by 3 dB.
A prior approach for reducing intermodulation increases the bias current to the amplifier. For example, a transistor capable of handling a higher power level is used. However, such transistors are generally more susceptible to noise and require a high bias current. A problem with such approaches is that certain applications, such as Coded Division Multi-Access (CDMA) cellular telephone systems, require very low noise and low bias current.
Therefore, while other designs may effectively address intermodulation for certain classes of radio receiver applications, they lack general applicability. A circuit arrangement that addresses the above identified problems would therefore be desirable.